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          ES6 模块
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        <h3 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h3><p>作为 JS 中最重要的代码组织模式，了解 ES6 模块也是很有必要的。在最近的阅读学习过程中，对于模块这部分也略有心得，在此记录一下习得的知识。</p>
<blockquote>
<p>知识点来自《你所不知道的 JavaScript 下卷》</p>
</blockquote>
<a id="more"></a>
<h3 id="旧方法"><a href="#旧方法" class="headerlink" title="旧方法"></a>旧方法</h3><p>在程序编写过程中，我们有时候需要一个包含内部变量和函数的外层函数，来实现面向接口式的编程。例如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 目标功能</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Hello</span>(<span class="params">name</span>) </span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">function</span> <span class="title">greeting</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'hello'</span> + name + <span class="string">'!'</span>);</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 公共 API</span></span><br><span class="line">  <span class="keyword">return</span> &#123;</span><br><span class="line">    greeting: greeting</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>每一次调用 Hello 我们都会生成一个新的实例，如果我们仅需要单例的话，此常见的变形便是闭包（IIFE）了，如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> hello = (<span class="function"><span class="keyword">function</span>(<span class="params">name</span>) </span>&#123;</span><br><span class="line">  <span class="function"><span class="keyword">function</span> <span class="title">greeting</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'hello'</span> + name + <span class="string">'!'</span>);</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 公共 API</span></span><br><span class="line">  <span class="keyword">return</span> &#123;</span><br><span class="line">    greeting: greeting</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)(<span class="string">'katty'</span>);</span><br><span class="line"></span><br><span class="line">hello.greeting(); <span class="comment">// hello katty !</span></span><br></pre></td></tr></table></figure>
<h3 id="新方法"><a href="#新方法" class="headerlink" title="新方法"></a>新方法</h3><p>“新方法”其实有很多，比如 UMD、AMD和CommonJS等，但是此处仅仅介绍 ES6 的模块方法，毕竟基于语言官方推荐的模块导出引入，必将成为未来的主流方法。</p>
<p>支撑起 ES6 模块的两个主要关键字是 export 和 import，此两方法必须出现在所有代码块和函数的外面（如：不能放在 if 方法内），下面详细介绍一下。</p>
<h4 id="导出方法"><a href="#导出方法" class="headerlink" title="导出方法"></a>导出方法</h4><p>export 关键字即表示模块的导出。导出的方式有很多种，比如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 命名导出</span></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">const</span> arrPos = <span class="number">1</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 默认导出</span></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="function"><span class="keyword">function</span> <span class="title">bar</span>(<span class="params"></span>) </span>&#123;&#125;</span><br></pre></td></tr></table></figure>
<p>对于一个 ES6 模块来说，没有 export 表示的任何变量或者函数，其都是作用域内部私有的。</p>
<p>此外，一个模块内（即一个 js 文件内）只有一个默认导出方法，但是可以有多个命名导出的方法，下面就介绍一下“命名导出”。</p>
<h5 id="命名导出"><a href="#命名导出" class="headerlink" title="命名导出"></a>命名导出</h5><p>首先我们得注意的是，命名导出导出的是变量的引用，而非值的拷贝。例如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// bar.js</span></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">let</span> num = <span class="number">2</span>;</span><br><span class="line">num = <span class="number">3</span>;</span><br></pre></td></tr></table></figure>
<p>对于之后引用 bar.js 文件的 num 属性时，我们获得的 num 值为 3。</p>
<p>此外，命名导出还有许多变体，例如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">baz</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// ES6 的简略写法</span></span><br><span class="line"><span class="keyword">export</span> &#123; foo, baz &#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 更改导出名称</span></span><br><span class="line"><span class="keyword">export</span> &#123; foo <span class="keyword">as</span> bar, baz &#125;</span><br></pre></td></tr></table></figure>
<p>既然有更改导出的名称的写法，那么我们能不能更改默认导出的绑定内容呢？答案是可以的。我将其称之为“动态默认导出”。</p>
<h5 id="动态默认导出"><a href="#动态默认导出" class="headerlink" title="动态默认导出"></a>动态默认导出</h5><p>所谓动态默认导出，即它能够动态的改变默认导出的内容而不影响其余代码的更改。例如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> foo = <span class="number">42</span>;</span><br><span class="line"><span class="keyword">export</span> &#123; foo <span class="keyword">as</span> <span class="keyword">default</span> &#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">let</span> bar = <span class="string">"hello world"</span>;</span><br><span class="line"></span><br><span class="line">foo = <span class="number">10</span>;</span><br><span class="line">bar = <span class="string">"cool"</span>;</span><br></pre></td></tr></table></figure>
<p>当倒入这个模块时， default 和 bar 会默认绑定到局部变量 foo 和 bar，即它们会暴露更新后的值 10 和 “cool”。</p>
<blockquote>
<p>注：导出时刻的值是无关紧要的，导入时候的值也是，绑定是活连接，所以重要的是访问这个绑定时刻的当前值。</p>
</blockquote>
<p>导出内容能够动态改变，那么这样能否减少我们对模块导入时的代码量呢？那么看看“动态模块导出”吧。</p>
<h5 id="动态模块导出"><a href="#动态模块导出" class="headerlink" title="动态模块导出"></a>动态模块导出</h5><p>对于公共文件夹内文件来说，初期文件不多的情况我们经常会这么引用：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> ModuleA <span class="keyword">from</span> <span class="string">'./common/module-a'</span>;</span><br></pre></td></tr></table></figure>
<p>然而，当文件越来越多时，这种写法就比较笨拙了，因此我们可以维护一个公共 index.ts 文件来中心化文件导出：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// index.ts</span></span><br><span class="line"><span class="keyword">import</span> ModuleA <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br><span class="line"><span class="keyword">import</span> ModuleB <span class="keyword">from</span> <span class="string">'./module-b'</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> &#123;</span><br><span class="line">  ModuleA,</span><br><span class="line">  ModuleB</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> &#123;</span><br><span class="line">  ModuleA,</span><br><span class="line">  ModuleB</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>当然，这种写法基本能解决模块引入问题，然而缺点是每次新增文件后，都需要改 3 处，变动的点相对太多了。因此为了减少改变的代码量，我们可以使用“动态模块导出”法：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 动态模块导出</span></span><br><span class="line"><span class="keyword">export</span> &#123; <span class="keyword">default</span> <span class="keyword">as</span> ModuleA &#125; <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br><span class="line"><span class="keyword">export</span> &#123; <span class="keyword">default</span> <span class="keyword">as</span> ModuleB &#125; <span class="keyword">from</span> <span class="string">'./module-b'</span>;</span><br></pre></td></tr></table></figure>
<p>这样每次变动后，我们只需要新增一行即可，代码的改变量就很少了。</p>
<h4 id="导入方法"><a href="#导入方法" class="headerlink" title="导入方法"></a>导入方法</h4><p>与 export 相同，import 导入同样也有相应的变体，比如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 全量导入</span></span><br><span class="line"><span class="keyword">import</span> ModuleA <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br><span class="line"><span class="keyword">import</span> &#123; <span class="keyword">default</span> <span class="keyword">as</span> ModuleA &#125; <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 部分导入</span></span><br><span class="line"><span class="keyword">import</span> &#123; foo &#125; <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 变名导入</span></span><br><span class="line"><span class="keyword">import</span> &#123; bar <span class="keyword">as</span> foo &#125;  <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br></pre></td></tr></table></figure>
<p>当然，我们还可以将默认导入和其他命名导入一起导入。加入我们有这么一个导出的模块，例如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="function"><span class="keyword">function</span> <span class="title">bar</span>(<span class="params"></span>) </span>&#123;&#125;</span><br></pre></td></tr></table></figure>
<p>那么导入这个模块的默认导出和它的命名导出的写法为：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> Foo, &#123; bar &#125; <span class="keyword">from</span> <span class="string">'./module-a'</span>;</span><br></pre></td></tr></table></figure>
<p>ES6 模块这些强烈建议的方法是，只从模块导入需要的具体绑定。如果一个模块提供了 10 个 API 方法，但是你只需要其中的 2 个，有些人坚信将所有的 API 绑定都导入进了是一种浪费。</p>
<p>因此他们提倡的是“窄导入”（没有 export default），这样除了代码更加清晰外，其另一个好处是使得静态分析和错误检测（比如意外使用了错误的绑定名称）更加健壮。</p>
<p>然而这种方法的缺点是比较繁复、每次新增东西时都得更新 import 方法（特别是需要全量导入的情况），因此 import 又有一种语法变体可以支持这种模块的导入，称为“命名空间导入”</p>
<h5 id="命名空间导入"><a href="#命名空间导入" class="headerlink" title="命名空间导入"></a>命名空间导入</h5><p>命名空间导入就是将模块内所有的导出内容（包括默认导出的内容）都归纳在一个命名空间内，如：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 导出模块</span></span><br><span class="line"><span class="comment">// foo.ts</span></span><br><span class="line"><span class="keyword">export</span> <span class="function"><span class="keyword">function</span> <span class="title">bar</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line"><span class="keyword">export</span> <span class="keyword">let</span> name = <span class="string">"myName"</span>;</span><br><span class="line"><span class="keyword">export</span> <span class="function"><span class="keyword">function</span> <span class="title">baz</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 导入模块</span></span><br><span class="line"><span class="keyword">import</span> * <span class="keyword">as</span> foo <span class="keyword">from</span> <span class="string">"foo"</span></span><br></pre></td></tr></table></figure>
<blockquote>
<p>注意，如果对应的导出模块有 default 导出时，使用 * as xxx 全量引入时，我们可以通过 xxx.default 来获取/使用默认导出的内容。</p>
</blockquote>
<h4 id="模块依赖环"><a href="#模块依赖环" class="headerlink" title="模块依赖环"></a>模块依赖环</h4><p>模块部分最令人疑惑的是 A 导入 B，然后 B 导入 A，这种情况是如何工作的。</p>
<p>首先“模块A”：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// A 模块</span></span><br><span class="line"><span class="keyword">import</span> bar <span class="keyword">from</span> <span class="string">"B"</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">x</span>) </span>&#123;</span><br><span class="line">  <span class="keyword">if</span> (x &gt; <span class="number">10</span>) <span class="keyword">return</span> bar(x - <span class="number">1</span>);</span><br><span class="line">  <span class="keyword">return</span> x * <span class="number">2</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>然后是“模块B”：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// B 模块</span></span><br><span class="line"><span class="keyword">import</span> foo <span class="keyword">from</span> <span class="string">"A"</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="function"><span class="keyword">function</span> <span class="title">bar</span>(<span class="params">y</span>) </span>&#123;</span><br><span class="line">  <span class="keyword">if</span> (y &gt; <span class="number">5</span>) <span class="keyword">return</span> foo(y / <span class="number">2</span>);</span><br><span class="line">  <span class="keyword">return</span> y * <span class="number">3</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>在 ES6 的模块下，改两声明处于不同的作用域内，因此其需要额外的工作来支持这样的循环引用。下面是粗略概念的意义上循环的 import 依赖如何生效和解析的过程：</p>
<ul>
<li>如果先加载“模块A”，第一步是扫描这个文件分析所有的导出，这样就可以注册所有可以导入的绑定。然后处理 import … from “B”。</li>
<li>引擎加载“模块B”之后，会对它的导出绑定进行同样的分析。当看到 import … from “A”，它已经了解 “A” 的 API，所以可以验证 import 是否有效。现在它了解 “B” 的 API，就可以验证等待的 “A” 模块中 import … from “B” 的有效性。</li>
</ul>
<p>现在让我们验证一下这两个模块：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 引用 foo</span></span><br><span class="line"><span class="keyword">import</span> foo <span class="keyword">from</span> <span class="string">"A"</span>;</span><br><span class="line">foo(<span class="number">25</span>); <span class="comment">// 11</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 引用 bar</span></span><br><span class="line"><span class="keyword">import</span> bar <span class="keyword">from</span> <span class="string">"bar"</span>;</span><br><span class="line">bar(<span class="number">25</span>); <span class="comment">// 11.5</span></span><br></pre></td></tr></table></figure>
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